1. Technical Field of the Invention
The present invention relates generally to an optical signal transmission system and an improved structure of a magneto-optical modulator for use in the same which utilizes the Faraday effect designed to modulate an optical beam up to a higher frequency.
2. Background Art
Most of external optical modulators employed in typical optical signal transmission systems utilize the electro-optical effect (i.e., Pockel""s effect). Particularly, most of optical signal transmission systems for use in optical communication employ optical waveguide modulators utilize the electro-optical effect of an LiNb03 crystal (e.g., Nishihara et al., xe2x80x9cOptical Integrated Circuitxe2x80x9d published by Ohm Company, pp. 298-304 (1985)). The optical modulators using the electro-optical crystal, however, experience dc drifts (e.g., J. Appl. Phys. Vol. 76, No. 3, pp. 1405-1408 (1994)) or optical damage and has a difficulty in maintaining the reliability for long use. Alleviating such a characteristic deterioration involves an increase in manufacturing cost.
In recent years, there have been proposed optical signal transmission systems which receive an electric wave through an antenna and apply it to an electro-optics modulator in the form of a high-frequency signal (e.g., Japanese Patent First Publication Nos. 4-172261 and 10-186189).
Magneto-optical modulators using the magneto-optical effect have been studied for a number of years (e.g., Appl. Phys. Lett. Vol. 21 No. 8, pp. 394-396 (1972)), but they are still not in practical use because their response frequencies are lower than those of electro-optics modulators and thus being researched for use as optical magnetic field sensors or current sensors (e.g., J. Appl. Phys. Vol. 53 No. 11, pp. 8263-8265 (1982) and National Technical Report, Vol. 38 No. 2, pp. 127-133 (1992)).
Japanese Patent First Publication No. 7-199137 teaches the use of an magneto-optical modulator as a polarization modulator in an optical signal transmission system. The response rate of the magneto-optical modulator is, however, as low as several tens kHz. U.S. Pat. No. 6,141,140 teaches the use of an optical isolator as a magneto-optical modulator, but its response rate is also low. This is because typical optical isolators are usually covered with a metal member or employ metal magnets for applying a dc magnetic field thereto, so that application of a high-frequency field will result in generation of the eddy current, which makes it difficult to apply a high-frequency field at several tens kHz or more from the outside of the optical isolator. The optical isolator is in practice employed as an optical modulator and has the disadvantage that changing the degree of the magneto-optical effect (i.e., the Faraday effect) using an external magnetic field causes the light to be returned to an unwanted direction (i.e., toward a light source).
In recent years, magneto-optical modulators designed to measure the current in a semiconductor electric substrate in which a dc bias field is applied to a magneto-optical crystal film are also researched (e.g., Appl. Phys Lett. Vol. 68 No. 25, pp. 3546-3548 (1996) and Extended Abstract (61th Annual Meeting, 2000), The Japan Society of Applied Physics, University of Tokyo, 2000, 4p-Q-4).
Most of typical optical signal transmission systems are designed to modulate the driving current to be applied to a semiconductor laser at higher frequencies or utilize an optical waveguide modulator exhibiting the electro-optical effect (i.e., the Pockel""s effect). Directly modulating the driving current applied to the semiconductor laser does not require a special modulator, thus providing the advantage that the optical signal transmission system will be simple in structure. It is, however, difficult to modulate the light emitted from the semiconductor laser at frequencies higher than several GHz. Additionally, actuating a driver of the semiconductor laser at higher frequencies may result in a failure in desired modulating operation or difficulty in transmitting an output far away because of laser chirp due to the high-speed modulation.
Further, in an optical signal transmission network consisting of a great number of optical fibers, an optical signal usually contains noises arising from multi-reflection from optical parts installed in each transmission line. In order to avoid this problem, a light source such as an LED having a wide emission spectrum is employed occasionally. The frequency band in which the LED can be energized is, however, on the order of 100 MHz (see Hiroo Yonetsu, xe2x80x9cOptical Communication Device Engineeringxe2x80x9d published by Kogaku Shoin, pp. 135-141 (1991)) thus requiring a special optical modulator for modulating an output of the LED at frequencies higher then 100 HHz.
Optical waveguide modulators utilizes the electro-optical effect. In this case, the Pockel""s effect is capable of high-speed modulation of a laser beam or light produced by an LED and does not encounter the problem of chirping, but faces, as described above, the problems of dc drifts and optical damage. Alleviating these involves an increase in manufacturing cost. In optical signal transmission systems in which an electric signal received by an antenna is used to modulate an optical beam (i.e., optical carrier wave), a modulator is usually installed in the open air and thus will have the problems of the dc drifts and optical damage. Further, most of optical waveguide modulators using the Pockel""s effect are designed for a single mode. It is usually difficult to produce a waveguide having a greater core diameter of several tens of xcexcm to several hundreds of xcexcm. The optical waveguide modulators, thus, encounter the problem in that it is difficult to modulate at high speeds an output of a LED which requires an optical fiber having a great core diameter for transmitting a sufficient quantity of light, an output of an optical amplifier which is increased greatly in power, or an output of a fiber laser (greater in core diameter than several tens of xcexcm.
Magneto-optical modulators utilizing the Faraday effect are also being researched which apply a dc bias field in parallel to a magneto-optical crystal film installed in a transmission line fabricated on a semiconductor substrate or a microstrip line and monitor the waveform of current flowing through the line. The structure which monitors the waveform of current flowing through the line on the semiconductor substrate (e.g., Appl. Phys. Lett. Vol. 68 No. 25, pp. 3546-3548 (1996)), however, faces the problem in that the unadjustment of impedance of the transmission line on the substrate causes the wave ringing. The structure does not use an optical fibers as an optical transmission line and is unsuitable for optical signal transmission systems. The other structure which measures the waveform of current flowing through the microstrip line (e.g., Extended Abstract (61th Annual Meeting, 2000), The Japan Society of Applied Physics, University of Tokyo, 2000, 4p-Q-4) has an analyzer disposed behind an optical fiber connected to an output of a magneto-optical element and poses the problem in that increasing the length of the optical fiber will cause a linear polarized light to experience random polarization in the optical fiber, thus resulting in a difficulty in modulating the intensity an output of the analyzer. Further, the above magneto-optical modulators are designed to apply the dc bias field to the magneto-optical crystal film in the same direction as that of application of a high-frequency field. The application of the dc bias field great enough to change the magneto-optical crystal film into a monodomain structure will cause the magneto-optical crystal film to be saturated magnetically, thus resulting in an decrease in magnitude of a modulated output signal or failure in outputting a modulated signal.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an improved structure of a magneto-optical modulator capable of modulation of an optical beam or carrier over a wide range.
It is still object of the invention an optical signal transmission system equipped with a magneto-optical modulator which is higher in reliability for an increased period of time and capable of transmission of an optical signal without DC drifts and optical damage.
According to one aspect of the invention, there is provided an optical signal transmission system. The optical signal transmission system comprises: (a) a light source emitting an optical beam; (b) a high-frequency signal generator producing a high-frequency signal; (c) a magneto-optical modulator modulating the optical beam emitted from the light source; (d) an optical fiber transmitting the optical beam modulated by the magneto-optical modulator; and (e) an optical receiver receiving the modulated optical beam transmitted through the optical fiber. The magneto-optical modulator includes a polarizer, a magneto-optical element, an analyzer, a dc field generator, a high-frequency field generator, and an impedance adjuster. The dc field generator works to apply a dc bias field to the magneto-optical element. The high-frequency generator is responsive to the high-frequency signal from the high-frequency signal generator to apply a high-frequency field to the magneto-optical element. The impedance adjuster works to adjust impedance of the high-frequency field generator for establishing effective transmission of the high-frequency signal to the high-frequency field generator.
The application of the dc bias field to the magneto-optical element and use of the impedance adjuster between the high-frequency signal generator and the high-frequency field generator enables the magneto-optical modulator to produce modulation at high speeds, which cannot be achieved by conventional magneto-optical modulators. The realization of such high-speed modulation is attributed to the facts that the adjustment of the impedance of the high-frequency field generator through the impedance adjuster serves to achieve the effective transmission of the high-frequency signal to the high-frequency field generator and the application of the high-frequency field causes a multi-domain structure of the magneto-optical modulator to be translated into a monodomain structure. Usually, a frequency response limit of movement of a domain wall between domains of a magneto-optical element lies within a range of several tens to several hundreds MHz, so that the domain wall does not respond at frequencies higher than that range. It is, thus, impossible to use such a magneto-optical element in an optical modulator for an optical signal transmission system required to respond at high speeds. This problem may be solved by the structure of the invention as described above. Specifically, the application of the dc bias field to the magneto-optical element results in conversion of the multi-domain structure to the monodomain structure, so that the domain wall disappears, thus resulting in disappearance of the movement of the domain wall that is a factor of determination of an upper limit frequency of modulation, thereby allowing the speed of the modulation to be increased.
In the preferred mode of the invention, the magneto-optical element is made of a multi-domain magneto-optical material. The dc field generator produces the dc bias field which is greater than a saturation field of the magneto-optical element.
The application of the dc bias field to the magneto-optical element is oriented at 90xc2x0xc2x130xc2x0 to a direction of application the high-frequency field to the magneto-optical element.
The direction of application of the high-frequency field to the magneto-optical element may be oriented in a direction of an axis of easy magnetization of the magneto-optical element. In a case where the magneto-optical element has a length, the direction of application of the high-frequency field to the magneto-optical element may be oriented substantially parallel to the length of the magneto-optical element, thereby minimizing the demagnetizing factor of the magneto-optical element.
The optical fiber is implemented by a graded index optical fiber or a polarization-maintaining fiber.
The optical beam is inputted at 90xc2x0xc2x115xc2x0 to an input surface of the magneto-optical element of the magneto-optical modulator.
The light source is implemented by one of an LED and a fiber laser.
An optical amplifier may be disposed between the light source and the magneto-optical modulator.
A lens may be disposed between the light source and the magneto-optical modulator.
A coupler may be provided which is connected to the magneto-optical modulator through the optical fiber. A mirror may be disposed on an end surface of the magneto-optical element opposite the input surface to produce a return of the optical beam modulated by the magneto-optical element. The coupler is also connected to the optical receiver to direct the return of the modulate optical beam to the optical receiver.
The high-frequency signal generator may be implemented by an antenna designed to receive the high-frequency signal in the form of an electric wave and transmit the high-frequency signal to the high-frequency field generator. The antenna may be one of a Yagi antenna, a loop antenna, and a parabola antenna.
According to another aspect of the invention, there is provided a magneto-optical modulator. The magneto-optical modulator comprises: (a) a polarizer to which an optical beam is inputted; (b) a magneto-optical element; (c) an analyzer outputting the optical beam from the magneto-optical element; (d) a dc field generator working to apply a dc bias field to the magneto-optical element; (e) a high-frequency field generator working to apply a high-frequency field to the magneto-optical element; and (f) an impedance adjuster working to adjust impedance of the high-frequency field generator.
In the preferred mode of the invention, the magneto-optical element is made of a magneto-optical material which has a multi-domain structure in the absence of application of the dc bias field. The dc field generator produces the dc bias field which is greater than a saturation field of the magneto-optical element.
The application of the dc bias field to the magneto-optical element may be oriented at 90xc2x0xc2x130xc2x0 to a direction of application the high-frequency field to the magneto-optical element.
The direction of application of the high-frequency field to the magneto-optical element may be oriented in a direction of an axis of easy magnetization of the magneto-optical element. In a case where the magneto-optical element has a length, the direction of application of the high-frequency field to the magneto-optical element may be oriented substantially parallel to the length of the magneto-optical element, thereby minimizing the demagnetizing factor of the magneto-optical element.
The magneto-optical element may be made of one of a bulk crystal, a polycrystal sintered body, a crystal film, and a composite containing resin and magneto-optical material dispersed in the resin.
The magneto-optical element may be made of a Bi-substituted garnet crystal film.
The frequency of the high-frequency field is 200 MHz or more.
The impedance adjuster may be implemented by an electric filter designed to allow a high-frequency signal of a preselected frequency to pass therethrough and be applied to the high-frequency field generator or by a resonator designed to have the high-frequency signal of the preselected frequency resonate.
The impedance adjuster may alternatively be implemented by an electric filter designed to allow high-frequency signals of at least two different frequencies to pass therethrough and be applied to the high-frequency field generator or by a resonator designed to have the high-frequency signals of the different frequencies resonate.
The dc field generator may be implemented by permanent magnets. The permanent magnets are each made of one of a ferrite material, a Smxe2x80x94Co based material, and a Ndxe2x80x94Fexe2x80x94B based material.
The dc field generator may alternatively consists of an electromagnet and a dc generator supplying current to the electromagnet.
The dc field generator may be geometrically designed so as to form a substantially closed magnetic circuit.
The polarizer, the magneto-optical element, and the analyzer may be fabricated in a single substrate.
The polarizer, the magneto-optical element, and the analyzer may be interposed between ferules arranged in alignment.
The high-frequency field generator may be installed on an end surface of the magneto-optical element.
The magneto-optical element is so oriented that the optical beam is inputted at 90xc2x0xc2x115xc2x0 to an input surface of the magneto-optical element.
The high-frequency field generator may be implemented by a coil whose minimum inner diameter is within a range of 10 xcexcm to 1000 xcexcm.
The magneto-optical element may alternatively be made of an optical wave guide made of a garnet crystal film.
The impedance adjuster may be made up of a TEM cell and a non-reflective terminator.
The impedance adjuster may be designed to resonate at a give frequency.
An electromagnetic wave shield casing may further be provided in which covers the polarizer, the magneto-optical element, the analyzer, and the high-frequency field generator.